fix(uploads): switch from S3 multipart POST to presigned PUT

Backblaze B2's S3-compatible endpoint does not implement the S3 POST
Object operation — every POST returns HTTP 501 regardless of URL form
(path-style or virtual-hosted-style). The previous multipart-POST flow
has been failing for every task-completion image upload.

Server-side companion change (honeyDueAPI master @7cc5448) replaces
PresignedPostPolicy with PresignHeader/PUT and renames the response
field from "fields" to "headers". This commit aligns both clients.

PresignUploadResponse model: field renamed `fields` → `headers`,
added `method` (default "PUT"). Both new fields have defaults so a
build talking to a stale server still decodes — albeit with empty
headers, which would then 403 at signature time. The server is
already on the new shape in prod.

iOS PresignedUploader.swift: dropped the ~70-line multipart body
builder and S3 form-field ordering logic. Replaced with a single PUT
request that applies server-supplied headers verbatim (skipping
Content-Length, which URLSession sets automatically and refuses to
override).

Android UploadApi.kt: same shape change. `postToStorage` →
`putToStorage`. Single Ktor `client.put()` with headers passthrough.
`uploadOne`'s `fileName` parameter kept for source compatibility but
marked @Suppress("UNUSED_PARAMETER") since PUT doesn't need it.

Verified end-to-end against api.myhoneydue.com:
  presign → PUT 12 bytes → HTTP 200 in 0.6s.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/com/tt/honeyDue/models/TaskCompletion.kt

@@ -34,15 +34,20 @@ data class PresignUploadRequest(
 /**
  * Presigned upload session — response from POST /api/uploads/presign.
  *
- * The client uses [uploadUrl] + [fields] to perform a multipart/form-data
- * POST directly to B2, then passes [id] back in the upload_ids[] field of
- * the next /api/task-completions/ or /api/documents/ create call.
+ * The client makes one PUT request to [uploadUrl] with the raw object
+ * bytes as the body and [headers] as the request headers. On success,
+ * pass [id] back in the upload_ids[] field of the next
+ * /api/task-completions/ or /api/documents/ create call.
+ *
+ * PUT (not POST) because B2's S3-compatible endpoint does not implement
+ * the S3 POST Object form upload (returns HTTP 501).
  */
 @Serializable
 data class PresignUploadResponse(
     val id: Int,
     @SerialName("upload_url") val uploadUrl: String,
-    val fields: Map<String, String>,
+    val method: String = "PUT",
+    val headers: Map<String, String> = emptyMap(),
     val key: String,
     @SerialName("expires_at") val expiresAt: String
 )

composeApp/src/commonMain/kotlin/com/tt/honeyDue/network/UploadApi.kt

@@ -5,7 +5,6 @@ import com.tt.honeyDue.models.PresignUploadResponse
 import io.ktor.client.*
 import io.ktor.client.call.*
 import io.ktor.client.request.*
-import io.ktor.client.request.forms.*
 import io.ktor.client.statement.*
 import io.ktor.http.*
 import io.ktor.utils.io.core.*
@@ -14,17 +13,16 @@ import io.ktor.utils.io.core.*
  * Three-step direct-to-B2 upload helper.
  *
  * Step 1: [presign] — call POST /api/uploads/presign on our API. Returns a
- *         B2 POST policy plus form fields the client needs to perform the
- *         direct upload.
- * Step 2: [postToStorage] — multipart/form-data POST straight to B2.
- *         Bytes never traverse our API server.
+ *         signed PUT URL plus the headers the client must send.
+ * Step 2: [putToStorage] — single PUT straight to B2. Bytes never traverse
+ *         our API server.
  * Step 3: caller invokes the relevant entity-creation endpoint
  *         (POST /api/task-completions/, POST /api/documents/) with the
  *         returned upload_id in the `upload_ids` field.
  *
- * iOS uses its own native equivalent (PresignedUploader.swift) for memory
- * reasons — Swift can stream a multipart body without buffering. Android
- * uses this Kotlin path which works fine for ≤10 MB images.
+ * iOS uses its own native equivalent (PresignedUploader.swift). Both paths
+ * use PUT because B2's S3-compatible endpoint does not implement the S3
+ * POST Object form upload (returns HTTP 501 for any POST).
  */
 class UploadApi(private val client: HttpClient = ApiClient.httpClient) {
     private val baseUrl = ApiClient.getBaseUrl()
@@ -61,38 +59,36 @@ class UploadApi(private val client: HttpClient = ApiClient.httpClient) {
     }
 
     /**
-     * Step 2 — POST `data` directly to B2 using the signed policy fields.
+     * Step 2 — PUT `data` directly to B2 using the signed URL + headers.
      *
-     * The S3 POST policy spec requires every signed field to appear before
-     * the file part, and `key` + `Content-Type` must match the policy
-     * exactly. Ktor's MultiPartFormDataContent preserves insertion order
-     * for the appended parts.
+     * The presign signature binds the headers exactly, so we send them
+     * verbatim. Content-Length is filled in automatically by Ktor from
+     * the body size, but we still pass through Content-Type which Ktor
+     * would otherwise default to application/octet-stream.
      */
-    suspend fun postToStorage(
+    suspend fun putToStorage(
         uploadUrl: String,
-        fields: Map<String, String>,
+        headers: Map<String, String>,
         data: ByteArray,
         contentType: String,
-        fileName: String,
     ): ApiResult<Unit> {
         return try {
-            val parts = formData {
-                // Stable order: signed fields first, then file. We rely on
-                // Ktor preserving the order in which append() is called.
-                fields.forEach { (k, v) -> append(k, v) }
-                append(
-                    key = "file",
-                    value = data,
-                    headers = Headers.build {
-                        append(HttpHeaders.ContentDisposition, "filename=\"$fileName\"")
-                        append(HttpHeaders.ContentType, contentType)
-                    },
-                )
+            val response = client.put(uploadUrl) {
+                // Apply server-supplied headers verbatim. Skip Content-Length
+                // — Ktor sets it automatically from the body and will refuse
+                // a manual override on most engines.
+                headers.forEach { (k, v) ->
+                    if (!k.equals("Content-Length", ignoreCase = true)) {
+                        header(k, v)
+                    }
+                }
+                // Defensive: ensure Content-Type is set even if the server
+                // omits it. The signed value (if present) takes precedence.
+                if (!headers.keys.any { it.equals("Content-Type", ignoreCase = true) }) {
+                    header(HttpHeaders.ContentType, contentType)
+                }
+                setBody(data)
             }
-            val response = client.submitFormWithBinaryData(
-                url = uploadUrl,
-                formData = parts,
-            )
             if (response.status.isSuccess()) {
                 ApiResult.Success(Unit)
             } else {
@@ -124,7 +120,7 @@ class UploadApi(private val client: HttpClient = ApiClient.httpClient) {
         category: String,
         contentType: String,
         data: ByteArray,
-        fileName: String,
+        @Suppress("UNUSED_PARAMETER") fileName: String,
     ): ApiResult<Int> {
         val presignResult = presign(token, category, contentType, data.size.toLong())
         val presigned = (presignResult as? ApiResult.Success)?.data
@@ -133,16 +129,15 @@ class UploadApi(private val client: HttpClient = ApiClient.httpClient) {
                 (presignResult as? ApiResult.Error)?.code,
             )
 
-        val postResult = postToStorage(
+        val putResult = putToStorage(
             uploadUrl = presigned.uploadUrl,
-            fields = presigned.fields,
+            headers = presigned.headers,
             data = data,
             contentType = contentType,
-            fileName = fileName,
         )
-        return when (postResult) {
+        return when (putResult) {
             is ApiResult.Success -> ApiResult.Success(presigned.id)
-            is ApiResult.Error -> postResult
+            is ApiResult.Error -> putResult
             else -> ApiResult.Error("Upload failed in unknown state")
         }
     }

iosApp/iosApp/Helpers/PresignedUploader.swift

@@ -4,15 +4,18 @@ import ComposeApp
 /// Three-step direct-to-B2 image upload.
 ///
 /// Flow:
-///   1. POST /api/uploads/presign → server returns a B2 POST policy + form
-///      fields scoped to a single object key with a content-length-range
-///      condition that B2 enforces at the protocol level.
-///   2. Multipart POST the bytes directly to B2, no API server in the data
-///      path. B2 rejects the upload if the bytes don't match the policy.
+///   1. POST /api/uploads/presign → server returns a signed PUT URL plus
+///      the headers (Content-Type, Content-Length) the client must send.
+///      The signature binds those headers — B2 rejects the upload if the
+///      bytes/headers don't match exactly.
+///   2. PUT the bytes directly to B2, no API server in the data path.
 ///   3. Caller passes the returned `uploadId` to /api/task-completions/ or
 ///      /api/documents/ via `upload_ids[]`. The server HEADs the object,
 ///      confirms the size, and creates the linked entity rows.
 ///
+/// We use PUT (not POST) because B2's S3-compatible endpoint does not
+/// implement the S3 POST Object form upload — every POST returns HTTP 501.
+///
 /// All errors map to `PresignedUploaderError` — the Swift call site can
 /// translate to user-facing copy without parsing nested HTTP details.
 enum PresignedUploaderError: Error, LocalizedError {
@@ -33,7 +36,7 @@ enum PresignedUploaderError: Error, LocalizedError {
             default:  return "Couldn't start upload (server returned \(status))."
             }
         case .uploadFailed(let status, _):
-            return "Upload failed (B2 returned \(status))."
+            return "Upload failed (storage returned \(status))."
         case .sessionError(let err):
             return err.localizedDescription
         }
@@ -95,13 +98,12 @@ final class PresignedUploader {
             contentLength: Int64(data.count)
         )
 
-        // Step 2: direct POST to B2
-        try await postToStorage(
+        // Step 2: direct PUT to B2
+        try await putToStorage(
             uploadURL: presigned.uploadUrl,
-            fields: presigned.fields,
+            headers: presigned.headers,
             data: data,
-            contentType: contentType,
-            fileName: fileName
+            contentType: contentType
         )
 
         return Int32(presigned.id)
@@ -146,7 +148,8 @@ final class PresignedUploader {
     private struct PresignResponse: Decodable {
         let id: Int
         let upload_url: String
-        let fields: [String: String]
+        let method: String?
+        let headers: [String: String]
         let key: String
         let expires_at: String
 
@@ -196,64 +199,39 @@ final class PresignedUploader {
         }
     }
 
-    // MARK: - Step 2: POST to B2
+    // MARK: - Step 2: PUT to B2
+    //
+    // The presign response includes the exact headers (Content-Type +
+    // Content-Length) that were signed. Send them verbatim — any deviation
+    // invalidates the signature and B2 will reject the upload.
+    //
+    // Content-Length is set automatically by URLSession from httpBody.count,
+    // so we don't manually echo it back; we still send Content-Type because
+    // URLSession will otherwise default it to application/x-www-form-urlencoded.
 
-    private func postToStorage(
+    private func putToStorage(
         uploadURL: String,
-        fields: [String: String],
+        headers: [String: String],
         data: Data,
-        contentType: String,
-        fileName: String
+        contentType: String
     ) async throws {
         guard let url = URL(string: uploadURL) else {
             throw PresignedUploaderError.uploadFailed(status: 0, body: "invalid upload url")
         }
 
-        // Build a multipart/form-data body with all policy fields followed
-        // by a single "file" part (S3 POST policy mandates the file part
-        // come last).
-        let boundary = "Boundary-\(UUID().uuidString)"
-        var body = Data()
-        let crlf = "\r\n"
-        let appendString: (String) -> Void = { s in
-            body.append(s.data(using: .utf8) ?? Data())
-        }
-
-        // Stable order: ensure "key" and "Content-Type" appear before the
-        // file part so the policy signature validates. Unspecified order
-        // for the rest — S3 accepts any.
-        let orderedKeys = ["key", "Content-Type", "policy", "x-amz-algorithm",
-                           "x-amz-credential", "x-amz-date", "x-amz-signature",
-                           "x-amz-meta-uid"]
-        var emitted = Set<String>()
-        for k in orderedKeys {
-            if let v = fields[k] {
-                appendString("--\(boundary)\(crlf)")
-                appendString("Content-Disposition: form-data; name=\"\(k)\"\(crlf)\(crlf)")
-                appendString(v)
-                appendString(crlf)
-                emitted.insert(k)
-            }
-        }
-        for (k, v) in fields where !emitted.contains(k) {
-            appendString("--\(boundary)\(crlf)")
-            appendString("Content-Disposition: form-data; name=\"\(k)\"\(crlf)\(crlf)")
-            appendString(v)
-            appendString(crlf)
-        }
-
-        // file part — must be last
-        appendString("--\(boundary)\(crlf)")
-        appendString("Content-Disposition: form-data; name=\"file\"; filename=\"\(fileName)\"\(crlf)")
-        appendString("Content-Type: \(contentType)\(crlf)\(crlf)")
-        body.append(data)
-        appendString(crlf)
-        appendString("--\(boundary)--\(crlf)")
-
         var req = URLRequest(url: url)
-        req.httpMethod = "POST"
-        req.setValue("multipart/form-data; boundary=\(boundary)", forHTTPHeaderField: "Content-Type")
-        req.httpBody = body
+        req.httpMethod = "PUT"
+        req.httpBody = data
+
+        // Apply server-supplied headers verbatim. Skip Content-Length —
+        // URLSession sets it automatically and will refuse to override it.
+        for (k, v) in headers where k.lowercased() != "content-length" {
+            req.setValue(v, forHTTPHeaderField: k)
+        }
+        // Defensive: ensure Content-Type is set even if the server omits it.
+        if req.value(forHTTPHeaderField: "Content-Type") == nil {
+            req.setValue(contentType, forHTTPHeaderField: "Content-Type")
+        }
 
         let (respBody, response): (Data, URLResponse)
         do {